The phoneme /æ/ is the open 'a' sound in 'mat', as opposed to the more constricted 'e' sound in 'met'. When we pronounce the vowel in 'mat', we can feel that the muscles of the mouth are open and relaxed, compared with a tighter, more closed position for for 'met'.
You need to practise listening to these two sounds. Don't try to relate them to sounds within your own language, as this will get you nowhere. Use online dictionaries and pronunciation sites to listen to the difference between the two sounds.
If you have an English-speaking friend or language partner, you can ask them to help you. As them to say 'met' and 'mat' a few times - or 'sat' or 'set' - in different combinations, for example, saying slowly 'mat mat met' or 'met mat met', to train your ear. Then they can do a little test for you. Ask your friend to say one of them clearly, either 'met' or 'mat', and try to guess which one it is. You'll soon train yourself to distinguish the difference.